Semiconductor devices typically include a semiconductor die mounted to a substrate. The die includes an active area in which circuitry is formed. The circuitry in the active area is electrically coupled to the substrate using connections at the surface of the active area. In a conventional device, the die is mounted so that the active area faces up, away from the substrate. In a flip-chip device, the die is mounted so that the active area is down, in proximity to the substrate.
In either conventional or flip-chip devices, detection of faults after fabrication of the semiconductor device is desirable. One such fault is a short. Both the existence of the fault and the position of the fault are desired to be determined. In conventional packages, shorts and other faults which generate heat may be detected using a liquid crystal. A thin layer of liquid crystal is poured over the circuits at the top of the die. When power is applied to the die, the liquid crystal changes phase over some hot spots in the circuits. Due to the phase change, the opacity and/or polarization of the liquid crystal changes over the hot spots. The centers of areas where the liquid crystal has changed phase mark the locations of shorts in the circuit. Consequently, the position of shorts in the circuit can be detected.
Although conventional liquid crystal detection can be used for circuits in conventional devices, this method has several drawbacks. Liquid crystal detection is primarily useful where the short is near a readily accessible surface. If the short is buried, heat is dissipated too rapidly with lateral distance within the semiconductor die for the liquid crystal to change phase. As a result, liquid crystal detection may not detect shorts in flip-chip devices or other cases in which the short is not very close to the surface on which the liquid crystal is placed. Conventional liquid crystal detection also biases the semiconductor die such that areas far from a short are below the temperature of the liquid crystal phase transition, while the short is above the temperature of the liquid crystal phase transition. This may be difficult or time consuming to accomplish. Finally, liquid crystal is carcinogenic. Consequently, health precautions must be taken during use of the liquid crystal.
Accordingly, what is needed is a system and method for detecting the positions of faults within a flip-chip device. The present invention addresses such a need.